Electronic publication (EPUB) is a free and open e-book standard that is adopted as a public standard to replace the Open e-Book standard that precedes September 2007 by International Digital Publishing Forum (IDPF). Since EPUB-based content is designed as reflowable content, it can be automatically optimized for the form and the size of a display device and provided to a user.
The specifications of EPUB include three definitions of open publication structure (OPS) 2.0, open packaging format (OPF) 2.0, and open e-book publication structure (OEBPS) container format (OCF) 2.0. The OPS is the uppermost-level definition of the EPUB format, and includes the definition of a modularization specification of EPUB content, the definition of a style sheet, the definition of limited use of extensible hypertext markup language (XHTML) or extensible markup language (XML), and so on. The OPF defines metadata generated in the OCF according to an OCF standard, an order of reading, and a mechanism for an information-searching operation, and the OCF specifies the general definition of a container technology of EPUB content.
Referring to the physical configuration of an EPUB file, EPUB-based content is in the form of a single file having the extension “.epub,” and several components (directories and files) are compressed into the single file by a compression mechanism. The root of an EPUB file includes a “mimetype.xml” file, a META-INF directory, and an OEBPS directory.
FIG. 1 is a diagram showing an OEBPS directory structure of an EPUB file. Referring to FIG. 1, the OCF defines that content to be actually read by users is located in an OEBPS directory in the form of an XHTML file.
As shown in FIG. 1, the OPF is embodied as a “content.opf” file in the OEBPS directory. The “content.opf” file includes metadata, the order of files containing content to be read by a user, a style to be presented, and navigational information that can be used by a device, such as an EPUB reader, or an application program.
Meanwhile, in connection with digital rights management (DRM) of content provided in the form of an e-book, such as EPUB-based content, IDPF has no standard enforcing DRM of EPUB-based content but has recommendations based on XML security standards of the world wide web consortium (W3C).
When encryption of e-book content is performed while the EPUB standard of IDPF is satisfied, it is recommended to generate an “encryption.xml” file containing key information and algorithm information for encrypting a META-INF directory according to the XML security standards of the W3C, generate a “signature.xml” file containing information on an e-signature for preventing forgery and falsification of content, and generate a “rights.xml” file for control of a content user's right. Here, the content of encryption, e-signature, and right control does not designate a single algorithm but suggests recommendations to use a specific algorithm and rights expression language.
The copyright protection of e-book content, such as EPUB-based content, becomes problematic because a content provider gives users rights to be provided with content in stages according to the users, or intends to prevent a non-allowed user from accessing content. However, as mentioned above, IDPF merely suggests recommendations for copyright protection and has no standardized algorithm for copyright protection. Therefore, there is a need for a copyright management algorithm that can be applied to EPUB.
An existing EPUB copyright protection system employs a method of encrypting a whole .epub file using one encryption key or separately encrypting components of an .epub file using one encryption key. Therefore, it is impossible to provide content in stages according to access rights.
Korean Unexamined Patent Application Publication No. 2003-0097465 discloses a DRM system that makes it possible to encrypt digital content using an encryption key and decrypt the digital content by getting a license, and US Unexamined Patent Application Publication No. 2010-0088746 discloses a system that maintains security by displaying only parts of a document according to access rights to confidential documents. However, all the existing technologies provide only security methods for general digital content and are irrelevant to a copyright protection algorithm for EPUB content.